Flaked phthalic anhydride



Patented Aug. 4,1931

use STATES B. .FQSTIEZR, OF BUFFALO, NEW Y PATE N'r OFFICE 03K, ASSIGNOB T NATIONAL ANILINE & N. Y, A CORPORATION OF NEW YORK FLAKED PHTHALIC ANH'YDRIDE Ho Drawing.

This invention relates to a method of purifying phthmic anhydride, particularly a crude or a partially purified phthalic anhydride which has been produced by the cataelytic oxidation of naphthalene or other organic substance in the vapor phase.

' In the production of phthalic anhydride by su jecting naphthalene in the vapor phase to partial or selective oxidation by means of to oxygen or an oxygen-containing gas in the presence of a suitable catalyzer' at an appropriate temperature, it is well known that the phthalic anhydride thus produced is usually contaminated with cetram impurities such as, for example, maleic anhydride, maleic acid and other unsaturated acids, benzoic acid, quinones and like bodies, phthaleins, tar, naphthalene, etc., and some water. It has been heretofore proposed to separate the W phthalic anhydride from. this mixture by sublimation, particularly in a current of air in conjunction with fractional condensation. Such a procedure, however, is expensive to carr out since it is dificult, if not impossible,

too tain the'greater part of the phthalic anhydride as a product which is uniform in character as to purity or texture, much ofv it must be 're-sublimed, and each sublimation gives rise to a considerable loss in yield caused by the current of air carryin the product beyond the condenser or condbnsing chamber. Moreover, the sublimed and even resublimed material is usuallystill impure in that it ordinarily contains one or more volaas above mentioned, it usually possesses an odor which is often unpleasant or undesirable, and

the product as such or after exposure to diffused sunlight in the presence of air for a considerabletime gives, when melted, a colored liquid. Of the impurities present, the quinones and related bodies, phthaleins and unsaturated aliphatic acidsv are especially persistent and difiicult to remove, if not impossible, by heretofore known methods.

' With ordinary distillation at atmospheric ressure of a crude or impure phthalic anhyride obtained in a catalytic we. as above m'entioned,it is found that the distillate is highly colored. It is sometimes possible to tile or sublimable impurities of the character Application filed January 21,1928. Serial Kc. 82,8517.

reduce and even temporarily remove the color by refluxing or autoclavin the crude or impure mixture prior to distillation. Distillation under a vacuum is ordinarily no more effective than at ordinary pressure. Slow fractional distillation gives a low ield. Moreover, the colorless products t us obtained upon exposure to diffused sunlight in the presence of air, e. g., two or more months, gradually become discolored or give, when melted, a colored liquid.

According to the present invention, phthalic anhydride, and particularly phthalic anhydride produced by' catalytic processes in the vapor phase oxidation of naphthalene, is purified by subjecting it to sublimation or to distillation, preferably the latter, in the presence of, or preferably preceded by a treatment with, a suitable agent or agents to form decomposition, oxidation or condensation products of or with the impurities. As such reagents, it has been found --that.sulfuric anhydride substances or compounds especially sulfuric acid, are particularly eflective. As sulfuric anhydride compounds, the inorganic acid sulfates and pyrosulfabes, as well as oleum and sulfur trioxide itself, may also be used. Other condensing agents, such as zinc chloride, caustic soda, copper sulfate, sodium plumbite, lead oxide, aluminum, zinc, tin, copper, an improvement in the character of the sub limate or distillate. There is thus obtainable in excellent yields a phthalic anhydride of higher purity and of a more uniform character than that obtained by other heretofore known methods, particularly known com-' merical methods.

The following specific example will further illustrate the invention, but it will be understood that the invention is not limited thereto. The parts are by Weight.

Ewample.lhere is charged into a reaction vessel about 30 parts 0 crude phthalic anhydride, obtained by the catalytic oxidation of naphthalene vapor with air, and a proximately 1 to 2 parts of 66 B. sulfuric acid. The charge is heated under atmospheric pressure to a temperature of about- 180-285 0., preferably about 250 0., and

etc., also effect maintained at this temperature for a period Y of about two to three hours or until the evolution of sulfur dioxide has nearly or substantially ceased. The molten mass is then run'into a still which is provided with a fractionating column and a condenser, which parts, together with the associated pipe lines,

the solidifying point of the distillate during distillation to avoid their becomng clogged. The distillate is cooled to about 150 C. in the condenser, and it is preferably returned from the condenser to the still until itcomes over as a clear water white liquid product, after which it is collected in suitable containers or molds where it solidifies; or the liquid distillate is brought into contact with any suitable or well-known flaking machine or apparatus and the phthalic anhydride. converted thereby into flakes or chips. It is preferable to employ a flaking a paratus which is enclosed or housed in or or to prevent or minimize losses by sublimation and a too. rapid cooling of the molten phthalic anhydride. The still is run until practically no more distillate is obtained, and it is brought to a high temperature near the end of the distillation to insure a substantially complete separation.

The residue left in the still has the appearance of afiufi'y, friable coke.

' Toward the end of the distillation, the distillate may come over colored andis collected separately. It may he returned to a subsequent batch, if desired, .thus preventing lossof material. Instead ofpermitting the liquid condensate from the condenser to re- Y turn to the still until a water white distillate is obtained, the

liquid condensate may be fractionally collected and the colorless por tion thereby separated from tions; but this ethod is not ordinarily advantageous since he colored portions by continued treatment soon yield colorless distillates. I

If, for any cause, not sufficientl pure it may be re-subjected to the process 0 t e invention for further purification.

, A phthalic anhydride can be thus obtained which in the solid form is white in color, melts into a liquid that is clear and colorless,

A and has a melting-point of 130.5 O. or

higher, e. posed to g., around 130.8 C. On being exdifiused sunlight in the presence of air over a considerable period of time, e. g.,'

two months or more, it undergoes substantially no change in color. It contains very little, if any, quinones or phthaleins, and not more than about one-quarter percent of unsaturated acids. It is completely soluble in hot water. Furthermore, the product bematerial.

the colored'por-' the purified product is cause of its uniformity in composition can be economy in packing, for example, more than twice as much of it-by weight can be-packed I nto a given space in comparison to sublimed A phthalic anhydride in the compact solid form, particularly in the form'of flakes or chips, and especially one which melts at or about 130.5 C. to a clear, colorless liquid, is regarded as a new product and as a part of the present invention.

The amount of sulfuric acid used depends upon the strength of the acid and the quality 7 of the phthalic anhydride being treated. If the sulfuric acid is insuificient in amount, there is incomplete purities and a colored, usually yellow, distillate is obtained. Too much sulfuric acid apparently'hasno other effect than a loss in yield. Ordinarily, the amount of sulfuric acid employed is about 2 to 1Q percent of the weight of crude phtha'lic anhydride treated, but more or less than this'amount may be destruction of the imused.

The heating preliminary to the distilla tion may be effected in the still, though a separate reactor is preferred on account of the evolution of gases and economies in operation, and it is found that, if the preliminary heating period be too short'or at too low a temperature, an incomplete separation and a yellow distillate results. For these reasons, a preliminary heat'ng period of two hours or more at a temperature ranging from about 250? C. to the boilin of phthalic anhydri e is desirable.

The fractionating column aids in preventing the discoloration of the distillate. Without its use, some impurities are apparently e'ntrained'and a colored product is apt to result..

The action of the sulfuric acid appears to be mainly. that of a condensing agent accompanied or followed by oxidation and carbonization of the impurities in view of the formation of sulfur dioxide and the coke that remains in the still. Whether a preliminary heating is desirable depends upon the character of the condensing agent employed or the gases or vapors that may result from the condensation and attendant reactions, but in the case of sulfuric acid or the sulfates, such reliminary heating results in producing an improved product, free from c0101. On the whole, as sulfuric acid has no apparent deleterious effect on the yield, and is readily available, inexpensive and in convenient liquidform, it is the preferred agent.

In the above example, instead of distilling the mass resulting from the treatment of the impure phthalic anhydride with sulfuric acid until the evolution of sulfur dioxide has nearly or substantially ceased, it may be subpoint (about 285 C.)'

'- pure phthalic anhydride with the condensing 'terial', e. g. phthalic anhy ide, at or below directly to the liquid state.

limed in any well-known manner, for example, by coohng the mass to about 160-170 C. and passing a current of air over or through it and condensing the sublimate in any suitable manner, for instance, fractional condensation; The major mid e portions of the sublimate are exceptionally pure. The present invention contemplates the use of diminished, ordinary. or super-atmospheric pressures in the treatment of the imagent, or in the distillation or sublimation operation, or both. If in the above example the distillation be carried out under reduce pressure, e. g, a gauge vacuum of about 25 to 30 inches 0 mercury, a phthalic anhydride substantially free from odor can be obtained.

Sublimation, according-t0 the present invention, consists in the va rizatlon of maits boiling polnt, and in thepresence or absence of. an inert gas, and the subsequent conversion or condensation of the resulting vapor directly to the solid state without p'assing through the liquid state, while distillation consists of the conversion of a substance at its boiling point from the liquid state to Q the seous or vapor state and the subsequent con ensation of the resulting vapor or gas The term fphthalic acid anhydride is intended to cover phthalic id or a mixture of the acid and anhydride. 'Ie the process has primarily in view the purification of a crude or a partially purified product obtained by the air oxidation of naphthalene I by catalytic methods, it isgenerally ap licable to impure 'phthalic acid anhydn e requiring the elimination of impurities, articularly impurities of the character alibve mentioned. Y

It may be pointed out that the various fractions as ordinaril obtained in the fractional condensation o tlie roduct roduced in the catalytic oxidation 0 naph ene may be separately or collectively purified by the process of the present invention.

In the claim, it will be understood that the term"ca'talytic phthalic anhydride refers to and is limited to a phthalic anhydride which has been prepared by the partial or selective oxidation of an organic bod parbicularly naphthalene,.in the vapor p ase in :he presence of oxygen or an oxygen-coritaln-v Ing gas anda suita 'le catalyst.

I claim: l ls a new product, flaked phthalic anhyn e. y

In testimony whereof I afiix m si ature,

. HAROLD B. F osi iia. 

